1. Field of the Invention
The present invention relates to a laser processing method of applying a pulsed laser beam to a workpiece to thereby process the workpiece.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped silicon substrate to thereby partition a plurality of regions where a plurality of circuits such as ICs and LSIs are respectively formed, thus preparing a semiconductor wafer having the plural circuits on the front side. The semiconductor wafer is cut along the streets to thereby divide the regions where the circuits are formed from each other, thus obtaining individual semiconductor devices. Further, an optical device wafer is prepared by forming a layer of gallium nitride base compound semiconductor or the like on the front side of a sapphire substrate, silicon carbide substrate, lithium tantalate substrate, or lithium niobate substrate. The optical device wafer is also cut along the streets to obtain individual optical devices divided from each other, such as light emitting diodes and laser diodes. These optical devices are widely used in electrical equipment.
As a method of dividing a wafer such as a semiconductor wafer along the streets, there has recently been proposed a method including the steps of applying a pulsed laser beam having an absorption wavelength (e.g., 532 nm, 355 nm, or 266 nm) to the wafer along the streets to thereby form a laser processed groove on the wafer along each street by ablation and next breaking the wafer along each laser processed groove (see Japanese Patent Laid-open No. 2005-353935, for example).